1. Field of the Technology
The present disclosure relates to earth-boring articles and other articles of manufacture comprising sintered cemented carbide and to their methods of manufacture. Examples of earth-boring articles encompassed by the present disclosure include, for example, earth-boring bits and earth-boring bit parts such as, for example, fixed-cutter earth-boring bit bodies and roller cones for rotary cone earth-boring bits. The present disclosure further relates to earth-boring bit bodies, roller cones, and other articles of manufacture made using the methods disclosed herein.
2. Description of the Background of the Technology
Cemented carbides are composites of a discontinuous hard metal carbide phase dispersed in a continuous relatively soft binder phase. The dispersed phase, typically, comprises grains of a carbide comprising one or more of the transition metals selected from, for example, titanium, vanadium, chromium, zirconium, hafnium, molybdenum, niobium, tantalum, and tungsten. The binder phase typically comprises at least one of cobalt, a cobalt alloy, nickel, a nickel alloy, iron, and an iron alloy. Alloying elements such as, for example, chromium, molybdenum, ruthenium, boron, tungsten, tantalum, titanium, and niobium may be added to the binder to enhance certain properties of the composite. The binder phase binds or “cements” the metal carbide regions together, and the composite exhibits an advantageous combination of the physical properties of the discontinuous and continuous phases.
Numerous cemented carbide types or “grades” are produced by varying parameters that may include the composition of the materials in the dispersed and/or continuous phases, the grain size of the dispersed phase, and the volume fractions of the phases. Cemented carbides including a dispersed tungsten carbide phase and a cobalt binder phase are the most commercially important of the commonly available cemented carbide grades. The various grades are available as powder blends (referred to herein as a “cemented carbide powder”) which may be processed using conventional press-and-sinter techniques to form the cemented carbide composites.
Cemented carbide grades including a discontinuous tungsten carbide phase and a continuous cobalt binder phase exhibit advantageous combinations of strength, fracture toughness, and wear resistance. As is known in the art, “strength” is the stress at which a material ruptures or fails. “Fracture toughness” refers to the ability of a material to absorb energy and deform plastically before fracturing. “Toughness” is proportional to the area under the stress-strain curve from the origin to the breaking point. See MCGRAW-HILL DICTIONARY OF SCIENTIFIC AND TECHNICAL TERMS (5th ed. 1994). “Wear resistance” refers to the ability of a material to withstand damage to its surface. Wear generally involves progressive loss of material, due to a relative motion between a material and a contacting surface or substance. See METALS HANDBOOK DESK EDITION (2d ed. 1998). Cemented carbides find extensive use in applications requiring substantial strength, toughness, and high wear resistance, such as, for example, in metal cutting and metal forming applications, in earth-boring and rock cutting applications, and as wear parts in machinery.
The strength, toughness, and wear resistance of a cemented carbide are related to the average grain size of the dispersed hard phase and the volume (or weight) fraction of the binder phase present in the composite. Generally, an increase in the average grain size of the carbide particles and/or an increase in the volume fraction of the binder in a conventional cemented carbide powder grade increases the fracture toughness of the formed composite. However, this increase in toughness is generally accompanied by decreased wear resistance. Metallurgists formulating cemented carbides, therefore, are continually challenged to develop grades exhibiting both high wear resistance and high fracture toughness and which are suitable for use in demanding applications.
In general, cemented carbide parts are produced as individual parts using conventional powder metallurgy press-and-sinter techniques. The manufacturing process typically involves consolidating or pressing a portion of a cemented carbide powder in a mold to provide an unsintered, or “green”, compact of defined shape and size. If additional shape features are required in the cemented carbide part that cannot be readily achieved by pressing or otherwise consolidating the powder, the consolidation or pressing operation is followed by machining the green compact, which is also referred to as “green shaping”. If additional compact strength is needed for the green shaping process, the green compact can be presintered before green shaping. Presintering occurs at a temperature lower than the final sintering temperature and provides a “brown” compact. The green shaping operation is followed by a high temperature treatment, commonly referred to as “sintering”. Sintering densifies the material to near theoretical full density to produce a cemented carbide composite and optimize the strength and hardness of the material.
A significant limitation of press-and-sinter fabrication techniques is that the range of compact shapes that can be formed is rather limited, and the techniques cannot effectively be used to produce complex part shapes. Pressing or consolidation of powders is usually accomplished using mechanical or hydraulic presses and rigid tooling or, alternatively, isostatic pressing. In the isostatic pressing technique shaping forces may be applied from different directions to a flexible mold. A “wet bag” isostatic pressing technique utilizes a portable mold disposed in a pressure medium. A “dry bag” isostatic pressing technique involves a mold having symmetry in the radial direction. Whether rigid tooling or flexible tooling is used, however, the consolidated compact must be extracted from the tool, and this limitation limits the compact shapes that can formed. In addition, compacts larger than about 4 to 6 inches in diameter and about 4 to 6 inches in length must be consolidated in isostatic presses. Since isostatic presses use flexible tooling, however, pressed compacts with precise shapes cannot be formed.
As indicated above, additional shape features can be incorporated into a compact for a cemented carbide part by green shaping a brown compact after presintering. However, the range of shapes that are possible from green shaping is limited. The possible shapes are limited by the availability and capabilities of the machine tools. Machine tools that may be used in green machining must be highly wear resistant and are generally expensive. Also, green machining of compacts used to form cemented carbide parts produces highly abrasive dust. In addition, consideration must be given to the design of the component in that the shape features to be formed on the compacts cannot intersect the path of the cutting tool.
Cemented carbide parts having complex shapes may be fabricated by attaching together two or more cemented carbide pieces using conventional metallurgical joining techniques such as, for example, brazing, welding, and diffusion bonding, or using mechanical attachment techniques such as, for example, shrink fitting, press fitting, or the use of mechanical fasteners. However, both metallurgical and mechanical joining techniques are deficient because of the inherent properties of cemented carbide and/or the mechanical properties of the joint. Because typical brazing or welding alloys have strength levels much lower than cemented carbides, brazed and welded joints are likely to be much weaker than the attached cemented carbide pieces. Also, since the brazing and welding deposits do not include carbides, nitrides, silicides, oxides, borides, or other hard phases, the braze or weld joint also is much less wear resistant than the cemented carbide materials. Mechanical attachment techniques generally require the presence of features such as keyways, slots, holes, or threads on the components being joined together. Providing such features on cemented carbide parts results in regions at which stress concentrates. Because cemented carbides are relatively brittle materials, they are extremely notch-sensitive, and the stress concentrations associated with mechanical joining features may readily result in premature fracture of the cemented carbide.
A method of making cemented carbide parts having complex shapes, for example, earth-boring bits and bit bodies, exhibiting suitable strength, wear resistance, and fracture toughness for demanding applications and which lack the drawbacks of parts made by the conventional methods discussed above would be highly desirable.
In addition, a method of making cemented carbide parts including regions of non-cemented carbide material, such as a readily machinable metal or metallic (i.e., metal-containing) alloy, without significantly compromising the strength, wear resistance, or fracture toughness of the bonding region or the part overall likewise would be highly desirable. A particular example of a part that would benefit from manufacture by such a method is a cemented carbide-based fixed-cutter earth-boring bit. Fixed-cutter earth-boring bits basically include several inserts secured to a bit body in predetermined positions to optimize cutting. The cutting inserts typically include a layer of synthetic diamond sintered on a cemented carbide substrate. Such inserts are often referred to as polycrystalline diamond compacts (PDC).
Conventional bit bodies for fixed-cutter earth-boring bits have been made by machining the complex features of the bits from steel, or by infiltrating a bed of hard carbide particles with a binder alloy, such as, for example a copper-base alloy. Recently, it has been disclosed that fixed-cutter bit bodies may be fabricated from cemented carbides employing standard powder metallurgy practices (powder consolidation, followed by shaping or machining the green or presintered powder compact, and high temperature sintering). Co-pending U.S. patent applications, Ser. Nos. 10/848,437 and 11/116,752, disclose the use of cemented carbide composites in bit bodies for earth-boring bits, and each such application is hereby incorporated herein by reference in its entirety. Cemented carbide-based bit bodies provide substantial advantages over machined steel or infiltrated carbide bit bodies since cemented carbides exhibit particularly advantageous combinations of high strength, toughness, and abrasion and erosion resistance relative to machined steel or infiltrated carbides.
FIG. 1 is a schematic illustration of a fixed-cutter earth-boring bit body on which PDC cutting inserts may be mounted. Referring to FIG. 1, the bit body 20 includes a central portion 22 including holes 24 through which mud is pumped, and arms or “blades” 26 including pockets 28 in which the PDC cutters are attached. The bit body 20 may further include gage pads 29 formed of hard, wear-resistant material. The gage pads 29 and provided to inhibit bit wear that would reduce the effective diameter of the bit to an unacceptable degree. Bit body 20 may consist of cemented carbide formed by powder metallurgy techniques or by infiltrating hard carbide particles with a molten metal or metallic alloy. The powder metallurgy process includes filling a void of a mold with a blend of binder metal and carbide powders, and then compacting the powders to form a green compact. Due to the high strength and hardness of sintered cemented carbides, which makes machining the material difficult, the green compact typically is machined to include the features of the bit body, and then the machined compact is sintered. The infiltration process entails filling a void of a mold with hard particles, such as tungsten carbide particles, and infiltrating the hard particles in the mold with a molten metal or metal alloy, such as a copper alloy. In certain bit bodies manufactured by infiltration, small pieces of sintered cemented carbide are positioned around one or more of the gage pads to further inhibit bit wear, In such cases, the total volume of the sintered cemented carbide pieces is less than 1% of the bit body's total volume.
The overall durability and service life of fixed-cutter earth-boring bits depends not only on the durability of the cutting elements, but also on the durability of the bit bodies. Thus, earth-boring bits including solid cemented carbide bit bodies may exhibit significantly longer service lifetimes than bits including machined steel or infiltrated hard particle bit bodies. However, solid cemented carbide earth-boring bits still suffer from some limitations. For example, it can be difficult to accurately and precisely position the individual PDC cutters on solid cemented carbide bit bodies since the bit bodies experience some size and shape distortion during the high temperature sintering process. If the PDC cutters are not located precisely at predetermined positions on the bit body blades, the earth-boring bit may not perform satisfactorily due to, for example, premature breakage of the cutters and/or the blades, excessive vibration, and/or drilling holes that are not round (“out-of-round holes”).
Also, because solid, one-piece, cemented carbide bit bodies have complex shapes (see FIG. 1), the green compacts commonly are machined using sophisticated machine tools, such as five-axis computer controlled milling machines. However, as discussed hereinabove, even the most sophisticated machine tools can provide only a limited range of shapes and designs. For example, the number and shape of cutting blades and the PDC cutters mounting positions that may be machined is limited because shape features cannot interfere with the path of the cutting tool during the machining process.
Thus, there is a need for improved methods of making cemented carbide-based earth-boring bit bodies and other parts and that do not suffer from the limitations of known manufacturing methods, including those discussed above.